In conventional organic EL light emitting devices, a voltage is supplied to an organic electroluminescence (organic EL) element including a light-emitting layer disposed between an anode (transparent electrode) and a cathode (back electrode) to cause light emission. The organic EL light emitting device has advantages in being light weight, thin, and low power consuming, and is used as a backlight for liquid crystal displays or as a planar illuminating device, for example JP-A-H8-315985.
Although the organic EL light emitting device has the above advantages, it also has the following problems.
Namely, because the organic thin film layers of the organic EL light emitting device, such as the organic light-emitting layer, or a support provided with the organic thin film layers have refractive indexes higher than the refractive index of air, total reflection of the emitted light tends to occur at an interface. Thus, the light utilization efficiency is no more than 20% of the total, meaning that most of the light is lost.
The organic EL light emitting device also has the problem of viewing angle dependency. Specifically, the light-emitting layer of the organic EL light emitting device includes a combination of a red light-emitting layer, a green light-emitting layer, and a blue light-emitting layer. These light-emitting layers have different refractive indexes, so that, when the light-emitting surface of the organic EL light emitting device is viewed at an angle, the light is separated according to wavelength at the interfaces between the light-emitting layers. When the light is separated according to wavelength, an optical path length difference is caused between the light-emitting layers, whereby the hue appears changed depending on the viewing angle. For example, when the organic EL light emitting device is viewed from the front, the optical path length change does not easily occur, and the change in color of the light emitted by the organic EL light emitting device does not easily occur. However, when viewed at an angle, the hue appears changed due to the optical path length change between the light-emitting layers.
As regards the problem of low optical utilization efficiency, proposals include providing a light outcoupling layer of a low refractive index material or placing a specific scattering member on the light outcoupling surface JP-A-2007-35313 and JP-A-2009-110930. However, these technologies do not provide an effective countermeasure for the wavelength separation of light.
Further, when the organic EL light emitting device is turned off, outside scenery or the like may be reflected on the surface of the organic EL light emitting device, adversely affecting the appearance of the surface.